The present invention relates to an electromagnetic flowmeter with a ceramic measuring pipe.
An electromagnetic flowmeter converts a flow rate of a conductive fluid flowing in a measuring pipe to an electrical signal by utilizing Faraday electromagnetic induction and measures the flow rate on the basis of the electrical signal. The electromagnetic flowmeter has many advantages in that no movable parts are used for measurement, no pressure loss occurs upon measurement, and a flow rate of a corrosive fluid or a fluid containing a slurry, which cannot be easily measured by other measuring instruments, can be measured. Electromagnetic flowmeters have been used in a variety of applications due to the above advantages.
A typical example of a measuring pipe used in a conventional electromagnetic flowmeter of this type is a metal pipe with a synthetic resin lining as shown in FIG. 1. Referring to FIG. 1, a metal measuring pipe 1 has flanges 1a to be coupled to pipes through which a fluid to be measured flows. The inner surface of the measuring pipe 1 and contact surfaces of the flanges 1a are covered with Teflon (PTFE) linings 2. A pair of excitation coils 3 of a U-shaped circumferential section are screwed on the outer surface of the measuring pipe 1. A pair of electrodes 4 are inserted in holes formed in the opposite wall surface portions of the pipe 1 which are rotated through 90.degree. from the circumferential centers of the coils 3. The electrodes 4 extend into the interior of the measuring pipe 1 and are in contact with the conductive fluid flowing therethrough. A meter case 5 is fixed on the flanges 1a of the measuring pipe 1 by screws 6. Ground rings 7 are screwed to the case 5 at the contact surfaces of the flanges 1a and are in contact with pipes through which the conductive fluid flows.
With this arrangement, when the conductive fluid flows through the measuring pipe 1, upon energization of the excitation coils 3, an electromotive force proportional to an average flow rate is generated across the electrodes 4 which are arranged perpendicularly to the direction of the magnetic field and the flow direction of conductive fluid, respectively. The electromotive force is measured to determine the corresponding flow rate of the conductive fluid. In this case, the inner surfaces of the ground rings 7 insulated by the linings 2 from the measuring pipe 1 are in contact with the fluid, the outer surfaces of the ground rings 7 are short-circuited to the case 5, and the conductive fluid is connected to the reference potential. Therefore, the electromotive force can be accurately extracted by the electrodes 4.
The conventional electromagnetic flowmeter can be properly operated when the measuring pipe 1 is a metal pipe, as described above. However, if the measuring pipe 1 is a ceramic pipe which is recently popular in favor of various advantages, holes for the screws 6 cannot be properly formed in the pipe and the manufacturing cost is increased. In addition, high mechanical strength cannot be expected.
In other conventional electromagnetic flowmeters with a ceramic measuring pipe each, the ceramic measuring pipe is fixed to a metal case by shrink fit. Japanese patent publication No. 58-501552 describes a typical example of a conventional electromagnetic flowmeter of this type, as shown in FIG. 2. Pipes 8 and 9 are coupled to flanges 1a at both open ends of a ceramic measuring pipe 1 through gaskets 10. A pair of excitation coils 3 are fixed on the outer surface of the measuring pipe 1. A metal case 5 is fixed to the flanges 1a of the measuring pipe 1 by shrink fit. A pair of electrodes 4 are fitted in holes formed in the wall of the measuring pipe 1 and are located at positions such that axes thereof are perpendicular to the direction of the magnetic field of the excitation coils 3 and to the flow direction of the conductive fluid. The operation of this flowmeter is the same as that in FIG. 1.
In the flowmeter shown in FIG. 2, after the excitation coils 3 and the electrodes 4 are mounted in position, the metal case 5 must be fixed to the measuring pipe 1 by shrink fit. Heat of shrink-fit is inevitably conducted to the excitation coils 3 and the like. Then, heat-resistant materials must be used for the coils 3 and the like. In addition, once the metal case 5 is shrink-fitted on the measuring pipe 1, the measuring pipe 1 cannot be replaced with a new one. As a result, the flowmeter itself must be replaced with a new one if a need for replacement of the pipe 1 arises.
Japanese Utility Model Prepublication No. 59-28219 describes another typical example of a conventional electromagnetic flowmeter with a ceramic measuring pipe, as shown in FIG. 3. A straight ceramic pipe 11 is shrink-fitted in the metal measuring pipe 1 with flanges 1a at both ends thereof. Pipes 8 and 9 are coupled to the measuring pipe 1 through gaskets 10. Excitation coils 3 and electrodes 4 are arranged in the measuring pipe 1 in the same manner as in FIG. 2. The metal case 5 is shrink-fitted on the flanges 1a of the measuring pipe 1. The operation of this flowmeter is the same as that in FIG. 1.
In the flowmeter shown in FIG. 3, the gaskets 10 as the seal surfaces are in contact with end faces of the metal flanges 1a when the pipes 8 and 9 are coupled to the measuring pipe 1. Therefore, even if the ceramic pipe 11 is fitted in the measuring pipe 1, the resistance to corrosion cannot be improved.